Demystifying heat pumps
The short version is just 5 bullet points:
Heat pumps are not a new idea, they were invented in the 19th Century.
Heat pumps move heat from one place to another, which has applications in both cooling and heating.
Most homes have one, in the form of a fridge.
Heat pumps can heat any home that a gas boiler can heat, and do so MUCH more efficiently.
So called ‘deep retrofit’ (very high levels of insulation) is NOT required before getting a heat pump.
Along with answers to 8 Myths about Heat Pumps, that’s all you need to know!
If you’re interested in learning a bit more, here is a slightly longer version, covered by these headings:
The origin of heat pumps
What is ‘Heat’ anyway?
Air-Source Heat Pumps
Efficiency of heat pumps is a game changer
Demystifying flow temperatures
Want to learn even more?
The Origin of Heat Pumps
Heat pumps were invented early in the 19th Century. They are able to move heat from one place to another. In a fridge, they move heat from inside the fridge to outside the fridge, using a substance (a fridgerant) that is liquid at very low temperatures, such as -50°C (minus 50°C) or lower.
The basic science explaining the workings of a heat pump was brilliantly demonstrated by Professor Andrea Sella in this short video demonstration (about a minute long) talking to the BBC’s Justin Rowlatt.
So the refridgerant can easily absorb heat from air that is at, say, -10°C. Indeed, if you take a sausage from the fridge that is pretty cold, say at 4°C, and move it to the freezer compartment set to -18°C, it will before long get to this lower temperature.
In an air conditioning unit (A/C), a heat pump moves heat from inside the house to outside to cool the interior. Heat pumps are everywhere! They are even now being used in tumble driers and in car heating/cooling systems.
In a paper ’On the Economy of Heating and Cooling of Buildings by Means of Currents of Air’ (Proceedings of the Philosophical Society, Vol.3, pp.269-272) “William Thomson (later Lord Kelvin) first proposed using heat pumps for space heating in 1852”, as noted in a history of heat pumps by Martin Zogg.
What is ‘Heat’ anyway?
Another scientific discovery of the 19th Century was that heat in substances is just the jostling of molecules. The hotter the substance, the faster this jostling. Hence it was termed the ‘kinetic theory of heat’. This jostling only stops at ‘absolute zero’, which is -273.15°C, which is 0 (zero) on the ‘absolute’ or Kelvin scale. So the freezing point of water is 273.15 Kelvin, and therefore there is lots of energy in air you may regard as very cold.
Air-Source Heat Pumps
When we talk about ambient energy/ heat this means energy ultimately derived from the Sun. This can then be extracted from the air, water or ground. But ground source heat pumps require a fairly large area of land, and costs more, so is not as accessible or even possible for most people. Similarly for water sourced heat pumps. So that is why Air Source Heat Pumps are likely to be the favoured option for most householders.
An Air Source Heat Pump (ASHP) has a fan that pulls air through it, to enable energy to be extracted from it, just as Lord Kelvin envisaged. Typically, it can pull 400 cubic metres of air through it in one minute. Imagine a room that is 10 metres x 10 metres and 4 metres high with air at 10°C that you have to heat up to 13°C in just one minute. That’s a lot of energy. The ASHP is extracting this amount of heat from the air on a continuous basis. It can take air that is flowing through it, and extract heat energy from it (which cools by a few degrees as you can tell by standing next to a heat pump), and then delivering the extracted heat to the house.
Most discussions of Air-Source Heat Pumps in the UK centre on ‘Air-to-Water’ systems that use traditional radiators (where water is used to distribute heat around a home), but it is important to be aware that there are also ‘Air-to-Air’ system that use warmed air to heat a home (they are discussed in this Nesta article ‘How air conditioning could help us tackle climate change’). Importantly, because ‘Air-to-Air’ systems are essentially like Air-Conditioning units operated in reverse, they can be used for cooling in summer as well as heating in winter. This could be especially important for smaller dwellings, such as flats.
Efficiency of heat pumps is a game changer, for bills and climate
Heat pumps are incredibly efficient, because they harvest so much energy from the ambient environment. For every unit of electrical energy used by an ASHP there are at least three units of heat energy produced (averaged over a year). This ratio is called the Seasonal Coefficient Of Performance (or SCOP) and in this case would be at least 3. This can also be expressed as an efficiency of 300%, averaged over a year, and often even better. One case study of a Grade 2 Listed building has achieved an average efficiency over a year of 330% (or SCOP of 3.3), and this was with minimal insulation measures. You can also learn about the experience of a heat pump installation in a Nailsworth three bedroom modern semi-detached home here.
With the turbulence that can result from one of the periodic flare ups in the Middle East, or elsewhere, sending gas prices skyward, the risks associated with continuing reliance on gas remain high. They’ve already hurt us in the UK following Russia’s invasion of Ukraine, because gas is purchase on international markets. It is an illusion to assume that cheap gas (which should attract substantial carbon taxes but doesn’t) will be as cheap as it has been previously. On the contrary. Whereas the Sun’s energy is free - whether directly in the form of the sun’s rays or indirectly in the form of wind that results from those rays heating the ground. Householders and communities (e.g. in the Stroud area) can also invest in renewables, and so not be entirely dependent on our Government or large energy companies for their energy needs.
The efficiency of heat pumps translates into a much reduced carbon footprint for heating, even using electricity from a grid that is not yet fully decarbonised. Even on a grid that is fully powered by gas powered generators, you’d be lowering your carbon footprint by at least two thirds using a heat pump, due to its efficiency. But of course in the UK the grid is already much better than that, and with each year that passes, with more wind and solar produced electricity on the grid, the carbon footprint of running a heat pump will progressively decrease, without you having to do a thing.
Demystifying flow temperatures
In a typical ‘air to water’ ASHP, the heat energy is used to heat water that is then pumped round the radiators in the house (or can be used to heat water in a hot water tank using a heat exchange coil).
Gas Boilers are often setup to send water that is at a high temperature such as 70°C round radiators. This is not necessary to heat a room to 21°C, and in fact, the lower the flow temperature the more efficient the system (whatever source of heating is used). Heating systems should use what is called ‘weather compensation’ to adjust the flow temperature to be optimal. The warmer it gets outside, the lower the flow temperature needed to deliver the target temperature needed.
Which is why it is very misleading to put your hand on the radiator to see if the system is ‘on’ or working ok. Your palm might be 34°C and the flow temperature might be just 40°C, so it won’t feel warm at all, but then look at a thermometer on the opposite wall, and see that the room is at its target temperature, say 21°C. Because 40 is greater than 21, and so the radiator is delivering heat to the room, and once the room has reached its target, the radiator keeps it there.
It’s a gentler way to heat the room, and far more efficient, compared to having radiators at 70°C.
In fact, modern building regulations require that the flow temperature should not exceed 55°C. Even in a large, poorly insulated home, you typically only need a maximum flow temperature of 50°C on the coldest day of the year. For most of the year the flow temperature will be much less.
For efficiency reasons (which translates to the bills you have to pay) the lower the better.
Want to learn even more?
Useful additional information on heat pumps comes from a number of trusted sources. For plain English and reliable information we recommend
Heat pumps: How do they work? is a gentle start, with the BBC and a great demo on the basic physics; and you can also learn about the Government’s Boiler Upgrade Scheme.
Heat Pumps - cut through the hot air provides practical advice from the Energy Saving Trust',
and they go a little bit further in their In-depth guide to heat pumps
For all aspects of heating, the heating hub provides lots of help and support
And Nesta - the UK's innovation agency for social good - has regular articles and reports on heating, and heat pumps. It is currently is running a Visit a Heat Pump scheme. Their report Heat pumps: a user survey found that “Heat pump users are highly satisfied with their heat pumps, considering them to be safe, reliable, quiet heat sources that are effective for space heating and producing hot water”, and is well worth a read.
And a recent report by the Green Alliance Decarbonising heat while addressing fuel poverty challenges the ‘fabric first’ mantra, the widely held belief that plenty of insulation is an essential precursor before getting a heat pump.
Deeper still
For those wanting to go a fair bit deeper, and perhaps thinking of taking courses to gain skills in heat pump system design and delivery, we recommend Heat Geek and
For those wanting to listen to longer form conversations from an award winning podcast site, covering a wide range of topics on renewable energy and low carbon heating, go to Betatalk.